This invention relates generally to sealing apparatus for heat treatment ovens, and, more particularly, to non-contact sealing apparatus for thin film treatment ovens to prevent leakage of gas into and out of the ovens at the film entry and exit openings.
In the manufacture of thin films of various materials, such as plastic, it is necessary to subject the films to various heat treatment. Thus, it is not uncommon in the manufacture of such films to convey the film either on a continuous belt or over an open draw through the oven.
In the course of such manufacturing processes, it is essential to maintain a controlled environment within the oven in order to assure that the plastic product retains or acquires the desired physical characteristics. Additionally, it is important to prevent leakage of solvents expelled from the plastic during heating in the oven into the surrounding plant area.
In connection with these requirements, sealing apparatus have been designed for use at the film entry and exit points of the ovens to prevent the entry of surrounding air into the ovens as well as to prevent leakage of the solvents from within the oven into the surrounding plant area. Such sealing apparatus have generally taken one of two forms, namely contact sealing apparatus and non-contact sealing apparatus. More particularly, such contact sealing apparatus wherein sealing members contact the film are not entirely satisfactory since it is not unusual for the film to be damaged due to improper contact with the seal material. On the other hand, sealing apparatus of the non-contact variety have previously not provided a reliable seal in that leakage of gases into and out of the oven has not been entirely eliminated. This is especially true where such seals have been provided on ovens which are intended to treat films having various thicknesses. In such cases, the precise dimensional tolerances which must be maintained in order to effect a reliable seal have not been possible to obtain. Further, it has not been uncommon to experience excessive gas leakage at the sides of the seals.
Another problem inherent in the use of non-contact or clearance type seals results from the thermal gradients present in the sealing apparatus caused by the difference in temperature between the hot oven interior and the cooler outside area. Such thermal gradients give rise to a lack of dimensional stability which is necessary to insure the maintenance of close seal clearance tolerances.